Contour-mode piezoelectric micromechanical resonators

ABSTRACT

A contour mode micromechanical piezoelectric resonator. The resonator has a bottom electrode; a top electrode; and a piezoelectric layer disposed between the bottom electrode and the top electrode. The piezoelectric resonator has a planar surface with a cantilevered periphery, dimensioned to undergo in-plane lateral displacement at the periphery. The resonator also includes means for applying an alternating electric field across the thickness of the piezoelectric resonator. The electric field is configured to cause the resonator to have a contour mode in-plane lateral displacement that is substantially in the plane of the planar surface of the resonator, wherein the fundamental frequency for the displacement of the piezoelectric resonator is set in part lithographically by the planar dimension of the bottom electrode, the top electrode or the piezoelectric layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/850,556, filed Aug. 4, 2010, which is a continuation of U.S. patentapplication Ser. No. 11/444,913, filed on May 31, 2006, now U.S. Pat.No. 7,791,432, which is a continuation-in-part of U.S. patentapplication Ser. No. 11/145,552, filed Jun. 2, 2005, now U.S. Pat. No.7,492,241, the entire contents of which are herein incorporated byreference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

This invention was made with Government Support under Grant (Contract)No. NBCH1020005 awarded by DARPA MTO/CSAC. The Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to micromechanical resonators. Inparticular, the present invention is related to a new class ofcontour-mode piezoelectric micromechanical resonators that can beemployed as building blocks in wireless communication components such asfilters and oscillators.

Recent demand in wireless communication for miniaturized, low-power,low-cost, on-chip and high-Q resonators to be employed in front-end RFfilters or as frequency references has focused research efforts towardsthe development of new vibrating micromechanical structures, capable ofsubstituting existing off-chip, bulky resonator technologies. Somepromising alternatives to currently adopted solutions (SAW or ceramicdevices) have been demonstrated (e.g., see, Li et al., IEEE MEMS,821-824 (2004); and Wang et al, IEEE MEMS, 641-644 (2004)) usingin-plane, electrostatically-transduced, micromechanical resonators madeof polysilicon or polydiamond. Although high quality factors have beenreported at ultra high frequency range (UHF), the exhibited impedancevalues are too high for these resonators to be directly coupled toantennas in RF systems. Also, the high temperature fabrication stepsinvolved with the deposition of the structural layers ultimatelycomplicate the integration of these devices with state-of-the-artmicroelectronic components.

Film Bulk Acoustic Resonator (FBAR) technology (e.g., see, Aigner etal., Transducers, 891-894 (2003); and Ruby et al., IEEE InternationalSolid-State Circuits Conference, 121-122) has proven itself as a validsolution to replace conventional RF filters, demonstrating relativelyhigh quality factors (Q˜2,500), and small (several Ω) impedances. Thefundamental frequency of these devices is set by the film thickness.This constitutes a major challenge to the manufacturing of FBARs. On onehand, in order to obtain reasonable yields, a thickness tolerance of0.1% is needed. On the other hand, multiple frequency selective arraysof resonators cannot readily be fabricated on a single chip, due to factthat the frequency of vibration for the devices is set by the filmthickness.

There is therefore a need for an improved resonator that does not sufferfrom the design disadvantages of currently available resonators.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed towards a new class of contour-modepiezoelectric micromechanical resonators that can be employed asbuilding blocks in wireless communication components such as filters andoscillators, and a method of making the same. The piezoelectricmaterials used for the fabrication of the micromechanical resonatorsincludes, for example, aluminum nitride (AlN), zinc oxide (ZnO),aluminum gallium arsenide (AlGaAs), Gallium Nitride (GaN), quartz andother piezoelectric materials. Other piezoelectric materials can includezinc-sulfide, cadmium-sulfide, lithium tantalate, lithium niobate, andother members of the lead lanthanum zirconate titanate family, andcombinations thereof. The use of contour modes, as opposed to FBARtechnology which employs a thickness mode, enables the fabrication ofarrays of microresonators with different frequencies on a single chip.In addition, the contour mode micromechanical resonators in accordancewith the embodiments of the present invention can be operated in airwithout significant performance degradation thereby reducing relatedpackaging costs. Low motional resistance and high quality factor arethus provided on the same chip while spanning a frequency range from MHzto GHz. The contour-mode piezoelectric micromechanical resonators inaccordance with the embodiments of the present invention also enable thefine and coarse tuning of their center frequencies directly on-chipwithout the need for additional post-processing steps.

In one embodiment, the present invention provides a contour modemicromechanical piezoelectric resonator. The resonator has a bottomelectrode; a top electrode; and a piezoelectric layer disposed betweenthe bottom electrode and the top electrode. The piezoelectric resonatorhas a planar surface with a cantilevered periphery, dimensioned toundergo in-plane lateral displacement at the periphery. The resonatoralso includes means for applying an alternating electric field acrossthe thickness of the piezoelectric resonator. The electric field isconfigured to cause the resonator to have a contour mode in-planelateral displacement that is substantially in the plane of the planarsurface of the resonator, wherein the fundamental frequency for thedisplacement of the piezoelectric resonator is set in partlithographically by the planar dimension of the bottom electrode, thetop electrode or the piezoelectric layer.

In another aspect, the present invention provides a method offabricating a contour mode micromechanical piezoelectric resonator bodyon a substrate. The method includes forming a patterned bottom electrodeabove the substrate; forming a piezoelectric layer above the bottomelectrode; forming a patterned top electrode on top of the piezoelectriclayer; forming an opening through the piezoelectric layer to the bottomelectrode; and etching the resonator body away from the substrate.

For a further understanding of the nature and advantages of theinvention, reference should be made to the following description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-(c) are exemplary diagrams of two rectangular-shaped and onedisk-shaped micromechanical resonator configured for vibrating in acontour-mode, in accordance with one embodiment of the presentinvention.

FIGS. 2( a)-(c) are exemplary diagrams showing in-plane contour modeshapes for a rectangular plate resonator, in accordance with oneembodiment of the present invention.

FIGS. 3( a)-(d) are exemplary diagrams of one embodiment of thefabrication process for the contour mode resonators in accordance withthe embodiments of the present invention.

FIGS. 4( a)-(c) are SEM images showing two rectangular-shaped and onedisk-shaped micromechanical resonator produced in accordance with thefabrication processes of FIGS. 3( a)-(d).

FIG. 5 is an exemplary diagram of another rectangular-shapedmicromechanical resonator configured for vibrating in a contour-mode, inaccordance with an alternate embodiment of the present invention.

FIGS. 6( a)-(b) are exemplary diagrams of a circular ring-shaped (a) anda square-shaped ring (b) micromechanical resonator configured forvibrating in a contour-mode, in accordance with one embodiment of thepresent invention.

FIGS. 7( a)-(d) are exemplary diagrams of an alternate embodiment of thefabrication process for the contour mode resonators in accordance withthe embodiments of the present invention.

FIGS. 8( a)-(d) are SEM images showing a circular ring-shaped and asquare-shaped ring micromechanical resonator produced in accordance withthe fabrication processes of FIGS. 7( a)-(d).

FIG. 9( a)-(b) show a ladder filter configuration incorporating thecontour-mode micromechanical resonators in accordance with theembodiments of the present invention (a); and a schematic representationof the electrical response of individual series and shunt resonators anda ladder filter made of the same (b).

FIG. 10( a)-(b) are SEM images showing a ladder filter made usingring-shaped micromechanical resonators (a) in accordance with theembodiments of the present invention; and a ladder filter made usingrectangular-shaped micromechanical resonators (b) in accordance with theembodiments of the present invention.

FIG. 11 is an exemplary oscillator circuit diagram that may beconfigured to include a contour-mode micromechanical resonator inaccordance with the embodiments of the present invention.

FIG. 12 is an exemplary diagram of a two-port stacked contour modemicromechanical resonator configured for vibrating in a contour-mode, inaccordance with one embodiment of the present invention.

FIGS. 13( a)-(d) are exemplary diagrams of one embodiment of thefabrication process for the contour mode resonator of FIG. 12.

FIG. 14 is an optical micrograph of micromechanical resonator withselectively patterned half-wavelength period electrodes.

FIG. 15 is a schematic of a resonator cross-section showing selectiveelectrode patterning and signal routing.

DETAILED DESCRIPTION OF THE INVENTION

Overview

The embodiments of the present invention are directed towards a newclass of contour-mode piezoelectric micromechanical resonators that canbe employed as building blocks in wireless communication components suchas filters and oscillators, and methods for making the same. Themicromechanical resonator includes a piezoelectric layer sandwichedbetween top and bottom electrodes, suspended in air and anchored in oneor more locations. Exemplary devices were demonstrated using AlN as thepiezoelectric layer. In addition, the piezoelectric materials that canbe used for the fabrication of the micromechanical resonators include,for example, aluminum nitride (AlN), zinc oxide (ZnO), aluminum galliumarsenide (AlGaAs), Gallium Nitride (GaN), quartz and other piezoelectricmaterials. Other piezoelectric materials can include quartz,zinc-sulfide, cadmium-sulfide, lithium tantalate, lithium niobate, andother members of the lead lanthanum zirconate titanate family, andcombinations thereof. The top or bottom electrodes may be made ofvarious materials including platinum (Pt), aluminum (Al), molybdenum(Mo), tungsten (W), titanium (Ti), niobium (Nb), ruthenium (Ru),chromium (Cr), doped polycrystalline silicon, doped AlGaAs compounds,ruthenium, gold, niobium, copper, silver, tantalum, cobalt, nickel,palladium, silicon germanium, doped conductive zinc oxide (ZnO), andcombinations thereof. An alternating current electric field appliedacross the top and bottom electrode induces mechanical deformations inthe plane of the piezoelectric layer (contour mode) via the d₃₁coefficient. At the device resonance frequency the electrical signalacross the device is reinforced and the device behaves as an equivalentelectronic resonant circuit (LC tank). The advantage of using amechanical resonator is the high quality factor of such a structure andthe consequent drastic reduction in its power dissipation. In addition,the devices in accordance with the embodiments of the present inventionare extremely small (in general 100 μm×200 μm) and can be used as afrequency reference in oscillator circuits and in band pass filters forminiature radios, wristwatch, and cellular phones.

Contour-mode piezoelectric micromechanical resonators in accordance withthe embodiments of the present invention have been demonstrated usingAlN technology. In one example, a thin AlN layer (e.g., 0.5 to 2 μmthick) is sputter deposited on a patterned Pt bottom electrode. A lowstress nitride layer is used as an insulating material between thebottom electrode and the silicon wafer. The insualting layer can be madefrom a material including silicon nitride, silicon dioxide, aluminumnitride, silicon carbide, titanium dioxide, polyimide dielectrics,hafnium dioxide, SU-8 photoresist, polymer dielectrics, and combinationsthereof. A top aluminum electrode is patterned on top of the AlN inorder to sandwich the piezoelectric material at the location whereactuation is desired. The structures are released by dry etching ofsilicon in xenon difluoride (XeF₂), thereby eliminating stiction forcesand significantly increasing yield. Alternatively, the structures can bereleased by a wet etching process. The fabrication steps occur at lowtemperature and offer the possibility to integrate the resonant deviceswith state-of-the-art microelectronic components. The patterning of thebottom electrode enables the on-chip coarse frequency tuning ofcontour-mode resonators. The acoustic impedance of Pt (significantlylarger than the one of AlN and Al) loads the resonator and shifts itscenter frequency. This frequency change can be defined directly at themask level without significant impact on the resonator performance. Forexample, two circular ring resonators having different electrode areas(coverage changes from 90% to 67.5% of the total resonator area) show afrequency shift of ˜6.8% from 232 MHz to 249 MHz. A fine-tuningmechanism of the resonator center frequency has also been demonstratedby applying a DC bias voltage across the piezoelectric film. Thecenter-frequency can be shifted either up or down by purelypiezoelectric means. A tuning range of ˜6kHz was obtained for a 23 MHzrectangular plate using a 30 V power supply. The combined use of on-chipmetal loading and DC biasing enables the efficient tuning of theresonant frequency of the piezoelectric devices without the need forpost-processing step and ultimately reducing the fabrication cost ofsuch devices.

Rectangular, diamond-shaped and circular plates, as well as circular andrectangular rings were successfully fabricated and operated.Contour-mode piezoelectric resonators were demonstrated to span afrequency range from 20 MHz to 1.15 GHz and can be extended up to 4 GHz.Quality factors of about 5,000 were obtained for rectangular platesexcited in dilation-type contour modes at 23 MHz with motionalresistance of 100 Ω. Ring-shaped resonators, excited inwidth-extensional mode shapes, exhibited quality factors as high as2,900 at a frequency of 473 MHz and a motional resistance of 80 Ω.

Contour Modes in Plates

FIGS. 1( a) and 1(b) show schematic views of AlN rectangular platesvibrating in dilation-type contour modes. FIG. 1( a) shows a two-port,AlN rectangular plate resonator 102 a with electrodes 103 a, 105 aplaced parallel to the plate length, L, and having an input port 107 aand an output port 109 a. FIG. 1( b) shows a two-port, AlN rectangularplate resonator 102 b with electrodes 103 b, 105 b placed parallel tothe plate width, W, and having an input port 107 b and an output port109 b. The resonators are shown to have a Pt bottom electrode and Al topelectrodes. Disposed between the top and bottom electrodes is the AlNpiezoelectric layer. A vertical electric field applied across the filmthickness induces extensional mechanical stress in the plane of the film(through the d₃₁ piezoelectric coefficient) and excites the structuresin a dilation-type contour mode. The two-port configurations wereimplemented by patterning the top electrodes symmetrically with respectto the length or the width of the plate. In order to minimize anchorlosses, the resonators were suspended by two quarter-wavelength tethers108 a, and 108 b.

Several in-plane contour mode shapes can be excited in these resonators(e.g., see, Holland, IEEE Transaction on Sonics and Ultrasonics,Su-15(2): 97-105 (1968)), but either electrode configurations or energyloss mechanisms limit the detectable mode shapes to the ones shown inFIG. 2. FIG. 2 shows dilation-type contour modes (obtained via a finiteelements analysis simulation) excited in a two-port MN 200×50 μmresonator. FIG. 2( a) shows a fundamental length-extensional mode shape,also referred to as a first mode herein; FIG. 2( b) shows a firstwidth-extensional mode shape, also referred to as second mode hereon;and FIG. 2( c) shows a second width-extensional mode showing effectivequarter-wavelength tethers. As is shown in FIGS. 2( a)-(c), theapplication of an alternating electric field across the thickness of thepiezoelectric resonator causes the resonator to have a contour modein-plane lateral displacement that is substantially in the plane of theplanar surface of the resonator.

The resonator can most effectively be excited in length-extensional(FIG. 2( a)) (e.g., see, Antkowiak et al., Transducers, 841-846 (2003))and width-extensional (FIGS. 2( b)-(c)) dilation-type contour modes. Aparameter that characterizes the relative strength of each of theresonances and sets the value of the equivalent motional resistance isthe electromechanical coupling, η_(em). Electromechanical coupling caneither be defined as the ratio of the output force to the input voltage,or the ratio of the output charge to the mechanical displacement. For asymmetric electrode topology such as the one used in FIGS. 1 a-b, η_(in)η_(out) are the same, specifically:

$\begin{matrix}\begin{matrix}{\eta_{em} = \frac{{\underset{A}{\int\int}}_{\;}^{\;}\left( {{d_{31}T_{31}} + {d_{31}T_{2}}} \right)\ {\mathbb{d}A}}{Z_{\max}}} \\{= \frac{\frac{2\; E}{1 - \sigma}d\; 31{\underset{A}{\int\int}}_{\;}^{\;}\left( {\frac{\partial u}{\partial x} + \frac{\partial v}{\partial y}} \right)\ {\mathbb{d}A}}{Z_{\max}}}\end{matrix} & (1)\end{matrix}$where T₁ and T₂ represent normal stresses in the x and y directionsrespectively, d₃₁ is the piezoelectric coefficient, u and v are thedisplacements in x and y directions (e.g., see, Johnson, MechanicalFilters in Electronics, New York, N.Y. Wiley (1983)), E is the Young'smodulus, and σ the Poisson ratio for AlN, A is the electrode area, andZ_(max) is the maximum displacement for the structure. Taking this intoaccount the electrodes were placed so that η_(em) is maximized.

Wineglass Contour Modes in Disks

FIG. 1( c) shows a schematic view of an AlN micromechanical diskconfigured to be excited in a wineglass contour mode. Due to the planesymmetry of AlN films, radial and compound mode shapes (e.g., see, Onoe,The Journal of the Acoustical Society of America, 28(6):1158-1162(1954)) can be effectively excited in thin circular disks through thed₃₁ piezoelectric coefficient. In order to minimize anchor losses, thedisk was pinned at two of its quasi-nodal points. FIG. 1( c) shows atwo-port, AlN disk-shaped resonator 102 c with electrodes 103 c, 105 cplaced as diagonally opposing quadrants, and having an input port 107 cand an output port 109 c.

A two-port configuration is implemented by having each of the two topelectrodes 103 c and 105 c cover a quadrant of the disk. By placing theelectrode in such a fashion, maximum electromechanical coupling isachieved. In this specific case η_(em) is given by:

$\begin{matrix}{\eta_{em} = \frac{\frac{2\; E}{\left( {1 - \sigma} \right)}d_{31}{{\underset{A}{\int\int}}_{\;}^{\;}\left\lbrack \left( {\frac{\partial u}{\partial v} + {\sigma\frac{u}{r}} + {\frac{1}{r}\frac{\partial v}{\partial r}}} \right) \right\rbrack}\ {\mathbb{d}A}}{Z_{\max}}} & (2)\end{matrix}$where E, σ, A and Z_(max) are the same as defined in equation (1), u andv represent the radial and tangential displacements (e.g., see, Onoe,The Journal of the Acoustical Society of America, 28(6):1158-1162(1954)), respectively, and r is the radial coordinate for the circularplate. Due to the isovoluminal nature of the mode shape and theelectrode covering just half of the plate top surface, theelectromechanical coupling for the disk is relatively smaller (˜8×) thanthat for AlN plates of comparable frequency. The resonator of FIG. 1( c)has been shown to be excited in a wineglass contour mode shape.

Fabrication Process

FIGS. 3( a)-(d) are exemplary diagrams of one embodiment of thefabrication process 300 for the contour mode resonators in accordancewith the present invention. Briefly, in step (a), a thin layer of AlN, alayer of Pt, and AlN film are deposited on a Si substrate. In step (b),trenches are opened through the AlN layers and the Pt layer by achlorine-based dry etching and ion milling, respectively. In step (c),openings to the bottom electrode are wet etched through the AlN layer ina hot (e.g., ˜160° C.) phosphoric bath. In step (d), the Al topelectrodes are evaporated and patterned by lift-off. Further details ofthe fabrication process are provided below. A three-mask,low-temperature (e.g., T_(max)<350° C.), post-CMOS compatible processwas used to fabricate rectangular and circular AlN plates. A thin (˜50nm) AlN buffer layer was deposited on a Si wafer to provide electricalisolation. A 1.5 μm AlN film was sandwiched between a bottom platinumelectrode and a top aluminum electrode. AlN is reactively depositedusing an Advanced Modular Sputtering Inc., AMS 2003 sputtering tool.Cl₂-based dry etching was used to pattern AlN and yielded fairlystraight (within 16° of vertical) sidewalls with an etch rate of ˜80nm/min. Oxide deposited by electron cyclotron resonance was used as ahard mask during the AlN etching step and the ion milling of the Ptlayer. Openings to contact the bottom electrode were wet etched throughAlN in a hot (e.g., ˜160° C.) phosphoric bath. The top Al electrode isevaporated and patterned so that maximum electromechanical coupling isobtained. The structures are released by dry etching of silicon in XeF₂.Alternatively, the structures can be released by a wet etching process.This novel dry release step for fully AlN resonators eliminates stictionforces and significantly increases yield compared to other processesthat use wet release techniques (e.g., see Humad et al., IEDM, 957-960(2001)). The deposition steps occur at low temperature (T_(max)˜350° C.when sputtering AlN) and thus can be integrated with state-of-the-artmicroelectronic components. FIG. 4 shows rectangular and circular plateresonators that were fabricated using the aforementioned process. FIG.4( a) shows an 80×20 μm rectangular plate with electrodes parallel tothe resonator length. FIG. 4 b shows a 50 μm radius AlN circular plateconfigured for excitation in a wineglass mode shape, and FIG. 4( c)shows an 80×20 μm rectangular plate with electrodes parallel to theresonator width.

For the fabrication of the device, either of the bottom electrode, thepiezoelectric layer and the top electrode or any other intermediatelayers can be formed using an evaporation-based process, a sol-gelprocess, a chemical vapor deposition process, a metallo organic chemicalvapor deposition process, a metallo oxide chemical vapor depositionprocess, and epitaxial process, an etching from bulk process, orcombinations thereof.

Characterization—Plate and Disk Resonators

The fabricated micromechanical resonators, such as those shown in FIG. 4above, were tested in a Janis micro-manipulated RF vacuum probe systemin SmTorr vacuum and at atmospheric pressure. Ground-Signal-Ground (GSG)probes from Picoprobe were used. The bottom Pt electrode was grounded,whereas each of the two top electrodes was used for either sensing ordriving the device under test. No interface circuitry between theresonator and the network analyzer was needed. Given the low impedancevalues, it was possible to directly measure the frequency response ofthe resonators using an Agilent 4195A network analyzer.

A. Frequency Response of AlN Rectangular Plates

Various rectangular plates with length to width ratios of 2 and 4 werefabricated and tested. The resonators varied in length from 80 to 200μm. All resonators, despite their size and electrode configuration,exhibited the fundamental length-extensional mode. The response invacuum of a 200×50 μm² plate, showed a Q of 3,280 and a motionalresistance, R_(x), of only ˜150 Ω when vibrating in its 1^(st) mode(FIG. 2( a)). Higher order modes in plates were excited with electrodesplaced parallel to the plate length and having a length to width ratioof 4. Q as high as 4,470 were obtained from the 2^(nd) mode (FIG. 2( b))of a 200×500 μm² plate at 80.57 MHz in vacuum. An 80×20 μm² plateachieved a measured frequency of 224.6 MHz with Q of 2,580 in vacuum,when vibrating in its 3^(rd) mode (FIG. 2( c)). The use ofquarter-wavelength tethers minimized the energy lost through the anchorsand enabled achieving quality factors higher than the one recorded forthe 1^(st) mode. It should be noted that the resonators in accordancewith the embodiments of the present invention were actuated in airwithout significant Q degradation. Recorded Q reduction was about 20-30%for all resonators. For example, Q of 2,000 in air and Q of 2,580 invacuum were recorded for an 80×20 μm² plate vibrating in its 3^(rd)mode. Other experimental measurements for dilation-type contour modes inAlN rectangular plates are summarized in a publication by the inventorsherein (e.g., see, Piazza et al., “Dry-Released Post-CMOS CompatibleContour-Mode AlN Micromechanical Resonators for VHF Applications,”Hilton Head Workshop, 38-41 (2004)). The recorded frequencies agreed towithin 3% of those obtained by a finite element analysis.

A 200×50 μm² resonant plate exhibited a linear TCF over a temperaturerange of 28-100° C. TCF values of ˜−26 ppm/° C., ˜−25 ppm/° C., and ˜−22ppm/° C. were recorded for the same plate vibrating in its 1^(st),2^(nd) and 3^(rd) mode, respectively. Furthermore, the center-frequencyof a plate was tuned both up and down by purely piezoelectric means. Aconstant strain was induced in the resonator by superimposing a DCvoltage to the ac signal on the two top electrodes. This tuningmechanism resulted in a ±3 kHz linear tuning range for a 22.97 MHzrectangular plate using a 30 V power supply (slope ˜4.4 ppm/V). Such atuning scheme can be employed, for example, to implement low-poweractive frequency compensation for temperature variations of ±10° C.

B. Frequency Response of AlN Disks

50 μm radius AlN disks vibrating in a wineglass contour mode shapeexhibited Q values as high as 5,830 at a frequency of 43.26 MHz invacuum. A high quality factor of 3,700 was recorded in air for the sametype of resonators. Using fairly small tethers (e.g., 5 μm wide) andanchoring the disk at its quasi-nodal points—since tangentialdisplacement is non-zero at these locations—resulted in the highest Qfor contour modes in AlN plates. Despite its high Q disk resonators showa motional resistance (˜73 kΩ) much higher than the one recorded forrectangular plates. Although, having the electrodes over the wholeresonator surface could decrease the motional resistance, theequivoluminal nature of the mode shape makes its excitation difficult inAlN films. The same resonator exhibited a linear TCF of ˜−14 ppm/° C.for a temperature range of 28-100° C. While not being limited to aparticular theory, the smaller value of TCF compared to the one recordedfor rectangular plates may be a consequence of the isovoluminal modeshape.

FIG. 5 shows an alternative embodiment of a rectangular-shapedmicromechanical resonator 500 configured for vibration in itscontour-modes, in accordance with the embodiments of the presentinvention. The MEMS resonator 500 is shown to have a three-layerstructure, where a layer of piezoelectric material 504 is locatedbetween a bottom electrode 502 and a top electrode 506. FIG. 5 alsoshows that a pad region on the bottom electrode 503 and a pad region onthe top electrode 507 allow for the application of an alternatingelectric field across the thickness of the piezoelectric resonator 500,where the electric field is configured to cause the resonator to have acontour mode in-plane lateral displacement in the planar surface of theresonator. FIG. 5 shows the pad region 503 to be one two pads 503 thatare located one on either side of the top pad 507. In the embodiment ofFIG. 5, the pads 503, 507, that are used to apply an electric fieldacross the thickness of the resonator 500, are located on the same sideas of the resonator body. However, it should be noted that pads could beplaced on different sides of the resonator body. The geometry of thepiezoelectric layer 504, the geometry of the bottom electrode 502 andthat of the top electrode 506 each in part contribute to the fundamentalfrequency of the excitation of the piezoelectric resonator. Thegeometries of the planar dimensions of the bottom electrode, the topelectrode and the piezoelectric layer are patterned and setlithographically.

FIG. 5 shows that for one geometric configuration, the bottom electrode502 surrounds the resonator's three-layer structure. The three-layerstructure is connected with and supported by the bottom electrode 502via tether 509. Other than at the tether 509, the three-layer stack hasa free perimeter or periphery that enables the three layer stack to beexcited in its contour-mode or lateral/extensional modes. The embodimentshown in FIG. 5 is one of several geometric configurations within thescope of the embodiments of the present invention, including theconfigurations of FIGS. 1( a)-(c), FIGS. 4( a)-(c), as well variousother alternate embodiments described below, and variants andcombinations thereof. It should be noted that the various alternativegeometric configurations described herein are exemplary embodiments thatshow various possible configurations. The scope of the embodiments ofthe present invention is not limited solely to these exemplaryconfigurations.

Ring-Shaped Resonators

FIGS. 6( a)-(b) are exemplary diagrams of a circular ring-shaped (a) 602a and a square-shaped ring (b) 602 b piezoelectric micromechanicalresonator configured for vibrating in a contour-mode, in accordance withan alternative embodiment of the present invention. The resonator 602 ahas a three layer stack for a circular-ring 604 shaped resonator body. Alayer of piezoelectric material 606 is located between a bottomelectrode 610 and a top electrode 608. The bottom electrode includes padregions 612 and the top electrode include pad region 614 that act asground and input electrodes respectively. Also shown in FIG. 6( a) isthat the three-layer ring-shaped structure 604 is connected with andsupported by the electrodes via tether 616. Other than at the tether616, the three-layer stack has a free perimeter or periphery thatenables the three layer stack to be excited in its contour-mode orlateral/extensional modes. FIG. 6 shows a structure where acircular-shaped ring resonator (602 a) is configured to be excited in aradial-extensional contour mode shape. FIG. 6 also shows an alternatestructure where a square-shaped ring resonator (602 b) is configured tobe excited in a width-extensional contour mode shape. Other than acircular or a square-shaped-ring structure, other shapes may also beused, including rectangle, a polygon, a circular annulus, a rectangularannulus, a polygonal annulus and combinations thereof.

FIG. 6 shows a schematic view of exemplary circular and squarering-shaped resonators that were excited in radial and width-extensionalcontour mode shapes, respectively. For one example, the resonator bodywas made out of a layer of AlN sandwiched between bottom Pt and top Alelectrodes. By applying an AC electric field across the film thickness,the active AlN piezoelectric layer undergoes (through the d₃₁piezoelectric coefficient) an in-plane lateral displacement that ismaximized at resonance. It is known that circular rings and, similarly,square rings vibrate in contour mode shapes whose fundamental frequencyis set primarily by the width of the ring (e.g., see, Li. et al., IEEEMEMS, 821-824 (2004); Bircumshaw et al., Transducers, 875-878 (2003)).

Finite element analysis has shown that Al and especially Pt electrodesaffect the resonator center frequency. Pt has a large mass density (6.5times that of AlN); its mass loads the resonator and decreases itsresonant frequency. Finite element modeling shows that this phenomenoncan be exploited to lithographically vary the center frequency of theresonator without substantially altering its performance. This feature,namely the dimensions and the geometry of the electrodes, which isunique to contour-mode resonators, may be used to tune the centerfrequency of resonators, for example, when employed in ladder filterstructures.

The asymmetry in the composition of the resonator's layers affects itspurely in-plane motion by introducing some warping. While not beinglimited to any theory, one could argue that this phenomenon canultimately degrade the quality factor and the transduction efficiency ofthe resonator by causing loss of the input energy in unwanted bendingand charge cancellation. Accordingly, a symmetric design may in generalbe preferable.

Another unique feature of the ring-shaped resonator designs (e.g., see,Bircumshaw et al., Transducers, 875-878 (2003)) is the possibility toarbitrarily select the value of motional resistance via the choice ofthe lateral area of the ring. In analogy with length-extensionalrectangular plates (e.g., see, Piazza et al., MEMS, 20-23 (2005)) themotional resistance, R_(x), of the rings can be approximately expressedby (for R_(ave)>>W):

$\begin{matrix}{R_{x} \approx {\frac{\pi^{2}}{8}\frac{T}{2\;\pi\; R_{ave}}\frac{\sqrt{\rho}}{{QE}_{P}^{3/2}d_{31}^{2}}}} & (1)\end{matrix}$

where R_(ave), T and W are the ring average radius, thickness and width;ρ and E_(ρ) are the mass density and modulus of elasticity of AlN. Thesame equation holds for a square ring where 2πR_(ave) is substituted bythe average perimeter of the ring. Therefore, this design permitschanges in R_(x) by varying R or L (length of square ring) withoutaffecting the resonator center frequency, and provides an extra degreeof freedom to the circuit designer (e.g., in oscillators and pass-bandfilters design).

In order to minimize energy loss to the substrate quarter-wave supportswere designed (e.g., see, Li et al., IEEE MEMS, 821-824 (2004)). Also inorder to investigate losses due to anchoring, the same type ofresonators were anchored by two quarter-wave, diametrically opposed,notched supports. Notching allows the anchors to be placed directly atthe nodal line of the ring and therefore reduces the interferencebetween the supports and the natural mode shape of the resonator (e.g.,see, Li et al., IEEE MEMS, 821-824 (2004)).

FIGS. 7( a)-(d) are exemplary diagrams of an alternate embodiment of thefabrication process 700 for the contour mode resonators in accordancewith the present invention. Briefly, in step (a) low stress nitride isdeposited by a low pressure chemical vapor deposition process (LPCVD)onto a silicon substrate, followed by a bottom electrode (e.g., Pt)patterning by lift-off and the sputter deposition of the AlNpiezoelectric layer. In step (b), the top electrode (e.g., Al) isdeposited and patterned by dry etching in a chlorine environment (Cl₂).In step (c), via access to the bottom electrode is opened through theAlN layer, by wet etching using a hot (e.g., 160° C.) phosphoric acid(H₃PO₄) bath. In step (d), the resonant device is dry etched in Cl₂ andthe structure is dry released in XeF₂. Alternatively, the structure canbe released by a wet etching process. Further details of the fabricationprocess are provided below.

For some examples, a four-mask, post-CMOS compatible process (i.e.,T_(max)<400° C.), such as the one described above was used to fabricatethe devices shown in FIG. 6 above. A 2 μm AlN film was sandwichedbetween a bottom platinum electrode and a top aluminum electrode. AlNfilms were sputter-deposited using a single-module AMS PVD sputteringtool. Highly crystalline films with rocking curve values below 2° wereobtained. The fabrication includes the patterning of the bottom Ptelectrode on a low stress nitride buffer layer in order to reduceparasitic capacitance and electrical losses. The top Al electrode waspatterned by dry etching. Cl₂-based dry etching was used to pattern AlNand obtain fairly straight sidewalls (e.g., 16° from the vertical).During the dry etch process the AlN film was masked by oxide depositedeither by electron cyclotron resonance or by low temperature chemicalvapor deposition techniques (T_(max)<400° C.). If LTO was used, the topAl electrode was protected by a thin (<30 nm) layer of Nb that wasremoved with the remaining oxide during a CF₄-based dry etch step. Thestructures were released by dry etching of silicon in XeF₂, therebyeliminating stiction forces and significantly increasing yield.Alternatively, the structures can be released by a wet etching process.As described above, the patterning of the bottom electrode enables theon-chip tuning of contour-mode resonators.

FIGS. 8( a)-(d) are SEM images showing circular ring-shaped andsquare-shaped ring micromechanical resonators produced in accordancewith the fabrication processes of Figs. FIGS. 7( a)-(d). FIG. 8( a)shows a SEM image of a single-supported circular-shaped contour-mode 20μm wide ring resonator. FIG. 8( b) shows a SEM image of asingle-supported square-shaped contour-mode 20 μm wide ring resonator.FIG. 8( c) shows a SEM image of a two-support circular-shapedcontour-mode 10 μm wide resonator and a detail view of notched supports;and FIG. 8( d) shows a SEM image of a two-support square-shaped contourmode 10 μm wide resonator and a detailed view of notched support. FIGS.8( c) and 8(d) show ring-shaped contour-mode resonators with notchedsupports. Notched supports were introduced in order to reduce anchorlosses. Although the top Al electrodes of these notched resonators werepatterned in such a way to permit a two-port configuration, thesedevices were successfully tested in a one-port configuration, for whichfewer spurious modes were observed.

Experimental Results—Ring-Shaped Resonators

The fabricated micromechanical resonators, similar to those shown inFIGS. 8( a)-(d), were tested in a Janis micro-manipulated RF probestation at atmospheric pressure. Ground-Signal-Ground (GSG) probes fromPicoprobe were used. S₁₁ parameters for each of the devices wereextracted and converted into equivalent admittances using an AgilentE8358A network analyzer. One-port S₁₁ parameter calibration (SOL) wasperformed using short and open reference structures directly fabricatedon the die under test, whereas a 50Ω resistor on a ceramic substrate wasused as a load reference.

Frequency Response—Ring-Shaped Resonators

A typical response of a circular ring microresonator with a singleun-notched support showed that it had a motional resistance as low as56Ω with Q of 2,400 in air for a 223.9 MHz resonator.

A comparison of identical resonators showed that notched supports didnot substantially improve the Q of the resonator, but they did reducethe interference between the anchors and the resonator motion,especially for resonators with smaller ring widths. Less mechanicalinterference translates into a cleaner electrical signal, with fewerspurious resonances. Other experimental results showed that a Q of 2,900was obtained for a circular ring resonator at 472.7 MHz with notchedsupports, a width of 10 μm and inner radius of 90 μm. The motionalresistance of this resonator was ˜84 Ω.

To further prove the less intrusive nature of notched supports, a 20 μmwide ring with 90 μm inner radius was excited in its 2^(nd) overtonereaching a frequency of 656.2 MHz with Q of 1,400 and R_(x)˜170Ω.

The value of the motional resistance of the resonators is controllableby changing the size of the inner radius. A 20 μm wide circular ringwith an inner radius of 40 μm exhibited a motional resistance of 204Ω at227.4 MHz. This reduction in size affects the quality factor of theresonators such that the Q value was reduced to 1,100 from 1,400.

By changing the width of top and bottom electrodes (without changing thewidth of the AlN ring) the center frequency of these resonators may betuned lithographically. For example, two circular ring resonators (20 μmwide and with 140 μm inner radius) having different electrode area(coverage changes from 65% to 95% of the total resonator area) show afrequency shift of ˜6.8% from 232 MHz to 249 MHz. The frequency shiftdoes not affect the strength of the signal because the majority of thestrain is located close to the nodal lines; therefore, the strain isefficiently transduced. A TCF of approximately −25 ppm/° C. was recordedfor the circular ring resonators.

Square-shaped micromechanical ring resonators were excited inwidth-extensional mode shapes. The typical response for a 10 μm wideresonator with inner ring length of 180 μm is shows that the resonatorswere able to reach a frequency as high as 475.3 MHz with respectable Qof 1,600 and R_(x)˜130Ω. A TCF of approximately −25 ppm/° C. wasrecorded for the square-shaped micromechanical ring resonators.Additional experimental results are provided in Piazza et al., “LowMotional Resistance Ring-Shaped Contour-Mode AlN PiezoelectricMicromechanical Resonators for UHF Applications,” IEEE MEMS 20-23(2005).

EXAMPLES Anchor Losses in Circular Rings

In order to study the effect of anchor losses on circular rings,un-notched tethers with three different widths (6,10 and 20 μm) werefabricated. The size of the supports slightly affects the resonatorcenter frequency, because of the increased stiffness, but no net changein Q was recorded. Resonators with two notched supports achieve Qfactors as high as those obtained for devices with just a singleun-notched support. The inventors herein believe that devices with asingle notched support could provide a higher Q.

Band Pass Filter Circuits

The contour-mode rectangular plate and ring-shaped piezoelectricresonators described above may be used as the building blocks for thevarious circuit configurations such as, for example, multiple-frequencyband pass filter and oscillator circuits. The embodiments of the presentinvention enable the formation of multiple-frequency, band pass filterson the same chip. FIG. 9( a) shows a ladder filter (e.g., L-network)configuration incorporating the contour-mode micromechanical resonatorsin accordance with the embodiments of the present invention (a). FIG. 9(b) shows a schematic representation of the electrical response ofindividual series and shunt resonators and a ladder filter of FIG. 9(a). The basic ladder filter configuration is composed of series andshunt resonators (FIG. 9( a)) to form an L network. Then these networkscan be cascaded to form more complicated multi-pole filters. Thesenetworks have been analyzed using an ABCD matrix approach (e.g., see,Lakin et. al., “Thin Film Bulk Acoustic Wave Filters for GPS,”Ultrasonic Symposium, 471-476 (1992)). For an initial proof of concept,high order filters (3rd and 4th) were built by simply cascading Lnetworks.

When designing band pass filters, it is desired to achieve a symmetricalgroup delay, low insertion losses and large out-of-band rejection. Inorder to provide a symmetrical group delay the filter termination,R_(term), is chosen to be the geometric mean of the series and shuntresonator reactances (e.g., see, Lakin et. al., “Thin Film Bulk AcousticWave Filters for GPS,” Ultrasonic Symposium, 471-476 (1992)), such that:

$\begin{matrix}{R_{term} = {\frac{1}{\omega_{C}}\sqrt{\frac{1}{C_{P}C_{S}}}}} & (1)\end{matrix}$

where ω_(c) is the filter center frequency, and C_(P) and C_(S) are theparallel capacitance of the shunt and series resonators, respectively.Maximum S₂₁ is obtained when the parallel resonance of the shunt branch,f_(PP), coincides with the series resonance of the series branch,f_(SS). In this condition, the insertion losses can be expressed as:

$\begin{matrix}{{I.L.} \approx {1 - \frac{n}{\omega_{C}C_{S}R_{term}k_{t}^{2}Q}}} & (2)\end{matrix}$

where n is the number of L networks, k_(t) ² is the effectiveelectromechanical coupling of AlN (e.g., see, Lakin et. al., “Thin FilmBulk Acoustic Wave Filters for GPS,” Ultrasonic Symposium, 471-476(1992)) and Q is the quality factor of the individual series resonators.In order to minimize losses, large k_(t) ² and Q are desired.Out-of-band rejection is set by the capacitance ratio between the seriesand shunt branch and by the number of L stages, such that:

$\begin{matrix}{S_{21{\_ Out}\text{-}{of}\text{-}{Band}} \approx \frac{1}{\left( {1 + \frac{C_{P}}{2\; C_{S}}} \right)^{n}}} & (3)\end{matrix}$

The fractional bandwidth of the filter is set by the distance betweenthe two zeros (the parallel resonance of the series branch, f_(PS), andthe series resonance of the shunt branch, f_(SP), respectively) of thefilter transfer function. Ultimately this translates into:

$\begin{matrix}{\frac{f_{PS} - f_{SP}}{f_{c}} \approx {\frac{8}{\pi^{2}}k_{t}^{2}}} & (3)\end{matrix}$

Therefore, the bandwidth of the filter depends on the electromechanicalcoupling coefficient and is theoretically limited to a maximum of ˜2.5%,unless other external elements are used.

Amongst the principal parameters on which the designer can act are thevalues of the parallel capacitance of the series and shunt resonators aswell as their ratio and the number of stages required. In order tominimize insertion losses and at the same time provide good out-of-bandrejection, a maximum number of 3 or 4 stages are selected. Thecapacitance ratio is chosen so that good out-of-band rejection can beobtained while maintaining high-Q resonators (e.g., in Piazza et al.,“Low Motional Resistance Ring-Shaped Contour-Mode AlN PiezoelectricMicromechanical Resonators for UHF Applications,” IEEE MEMS, 20-23(2005)). It was shown that Q degrades when the inner radius of the ringis made smaller. In the example filter circuits used herein capacitanceratios of 1 and 0.67 were used.

A four-mask, low-temperature, post-CMOS compatible (T_(max)<400° C.)process, such the process of FIGS. 7( a)-(d) was used to fabricate thefilter devices. The process flow is substantially the process of FIG. 7(a)-(d), and used micromachining technique to manufacture high qualityAlN resonators with high yield. For example, a single resonator has a 2μm AlN film sandwiched between a bottom platinum electrode (˜100 nmthick) and a top aluminum electrode (˜175 nm thick). AlN films weresputter-deposited using an AMS PVD tool and exhibit rocking curve valuesas low as 1.3° on Pt seed layers. The difference with respect to thefabrication process described in FIGS. 7( a)-(d) involves performing thewet etch of AlN to access the bottom electrode before the deposition andpatterning of the top Al electrode. This step is used to implement aladder filter topology, for which contact between the top and bottomelectrodes is required.

As it was shown in FIG. 9( b), the parallel resonance of the shuntbrunch should coincide with the series resonance of the series branchfor the filter to work properly. The shift can be defined directlyduring the processing step used for patterning the Pt electrode. The Ptelectrode has a very large mass density, about 6.5 times the one of AlN.By removing small amounts of Pt the center frequency of the resonatormay be raised. Being most of the strain (and consequently charge) andconcentrated in the middle of structure, small amounts of Pt arelithographically removed from the edges of the microstructures (FIG. 10)without affecting the overall performance of the resonators. The loadingmechanism was experimentally verified and may be explained by ananalytical model based on vibration techniques (e.g., see, Graff, “WaveMotion in Elastic Solids,” Dover Publications, Inc., New York). Theexperiments showed that for a 100 nm thick Pt the center frequency of a90 μm inner radius ring structure shifts by ˜7000 ppm for each μm of Ptthat is removed from the width of the structure, rectangular plateshifts by 500 ppm for each μm of Pt that is removed from the length ofthe structure.

FIG. 10( a) shows SEM images showing a ladder filter made usingring-shaped micromechanical resonators in accordance with theembodiments of the present invention. FIG. 10( a) shows a SEM view of aladder filter made from eight electrically cascaded ring-shapedresonators and a detailed view of the ring and the lithographicallydefined variation in the bottom electrode width used to tune the centerfrequency of the series resonators. FIG. 10( b) shows SEM images of aladder filter made using rectangular-shaped micromechanical resonatorsin accordance with the embodiments of the present invention. FIG. 10( b)shows a SEM view of a ladder filter made from eight electricallycascaded rectangular plate resonators and a detailed view of the singlerectangular resonator and the lithographically defined variation in thebottom electrode length used to tune the center frequency of the seriesresonators.

An alternate method for achieving the frequency shift is by changing thedimensions of the inner radius of the rings. By having a ring in theshunt branch with a diameter larger than the one in the series branch,the resonance frequency of the resonators in the shunt branch can belowered, while increasing the out-of-band rejection.

Experimental Results—Filters

The fabricated micromechanical filters were tested in a RF probe stationat atmospheric pressure. Ground- Signal-Ground (GSG) probes were used.Two-port S-parameter calibration (SOLT) was performed using short, openand through reference structures directly fabricated on the die undertest, whereas a 50Ω resistor on a ceramic substrate was used as a loadreference. Full S-parameter matrices were extracted for each filterusing an Agilent E5071B network analyzer. No external terminations wereconnected to the device under test. The network analyzer is used toautomatically change the terminations and compute the filtertransmission spectrum.

Eight rings, all with an inner radius of 90 μm and 20 μm width, wereelectrically cascaded in a ladder structure. The frequency of the seriesand shunt branches were shifted by ˜0.3%. This filter showed fairlymoderate insertion losses of −7.9 dB at 236.2 MHz, an out-of-bandrejection of 26 dB and a 20dB shape factor of 2.79. The filter does notsuffer from any other spurious resonance. As described above, thefrequency shift was also defined by changing the size of the innerradius of the rings in the shunt branch (inner radius of 140 μm was usedin the shunt branch and 90 μm in the series branch). Up to six ringswere connected in this configuration.

Four, six and eight 200 μm long and 50 μm wide rectangular plates weretested in a ladder configuration as well. Again, the frequencies wereshifted by about 0.3%. Insertion losses as low as −4 dB were recorded at93.2 MHz; out-of-band rejection of 27 dB were achieved. The resultsshowed that a second band pass filter exists due to thelength-extensional mode shape present in the plate. This mode may bepushed further down in frequency by changing the aspect ratio of themicrostructures. The results for four and six rectangular resonantfilters are summarized in Table I.

TABLE I Response parameters of band pass filters built using rectangularplate resonators. Number of f_(c) BW_(3 dB) I.L. 20 dB R_(term)Resonators [MHz] [kHz] [dB] S.F. [kΩ] 4 93.5 456 −2 N.A. 1.5 6 93.1 332−4.7 2.7 2

In addition to the L network arrangement for the ladder filter formed byelectrically cascading a network of resonators, other networkarrangements, including Pi, T, L, or a lattice network configurationsmay be used. Furthermore, filter networks may be a part of a band passfilter circuit formed using a series of either of the Pi, T or Lnetworks.

The examples of the filter circuits in accordance with the embodimentsof the present invention show that using a novel and disruptive MEMSresonator technology based on the excitation of contour mode shapes inAlN microstructures, band pass filters at 93 and 236 MHz were produced,by electrically cascading up to eight resonators in a ladder structure.These filters showed a good performance, being characterized by lowinsertion losses (4 dB at 93 MHz), large close-in and out-of-bandrejection (˜40 dB and ˜27dB, respectively, for a 93 MHz filter) andfairly sharp roll-off with a 20dB shape factor of ˜2.2. The filtersproduced in accordance with the embodiments of the present invention areapproximately 30 times smaller than existing SAW technology, commonlyused in the IF bands for cell phones. In addition, with a temperaturecoefficient of −25 ppm/° C., they have 40% lower temperature sensitivitythan SAW filters.

The MEMS resonator technology based on the excitation of contour modeshapes in piezoelectric microstructures in accordance with theembodiments of the present invention may be used to form filter circuitssuch as those described above. In addition, the contour-modepiezoelectric resonators in accordance with the embodiments of thepresent invention may also be used as part of other circuits, such as,for example, an oscillator circuit (e.g., a Pi-network). FIG. 11 is anexemplary oscillator circuit 1100 diagram that may be configured toinclude a contour-mode micromechanical resonator 1102 in accordance withthe embodiments of the present invention.

In addition to the resonator embodiments described above, an alternativeresonator embodiment includes an interleaved electrode made of a metallayer that is interleaved between the top and bottom electrode. Thisinterleaved meal layer is configured for use as a ground electrode, amass loading element or as a means for applying a potential. Theinterleaved electrode can be made from a material including aluminum,platinum, tungsten, molybdenum, ruthenium, chromium, gold, titanium,doped polycrystalline silicon, and combinations thereof.

FIG. 12 is an exemplary diagrams of a two-port stacked contour modemicromechanical resonator configured for vibrating in a contour-mode, inaccordance with one embodiment of the present invention.

FIG. 12 shows a schematic representation of a two-port stackedcontour-mode AlN piezoelectric resonator. The device can be formed bytwo aluminum nitride layers, each sandwiched between two electrodes(platinum or aluminum) and can vibrate in a contour mode shape. Avertical electric field can be applied to one of the two layers toinduce in plane dilation of the plate through the d₃₁ piezoelectriccoefficient. The rectangular plate can be excited into two fundamentalmodes, either a length-extensional or a width-extensional mode, beingthe center frequency set either by the width or length of the plate,respectively. At resonance, the large strain induced in the plateproduces a charge that can be piezoelectrically sensed by the otherlayer. The two-port topology permits the physical separation of theinput electrode from the output electrode, therefore reducing parasiticfeed through. In this device because the overall surface of the plate iselectroded, a maximum electromechanical coupling can be achieved. Bymoving from a horizontal two-port configuration to a vertical two-porttopology the direct feed through that is experienced through the device[e.g., see G. Piazza, et al., Hilton Head Workshop, pp. 38-41, June2004] and that would have been extremely problematic at very highfrequencies can be eliminated or reduced.

FIGS. 13( a)-(d) are exemplary diagrams of one embodiment of thefabrication process for the contour mode resonator of FIG. 12. As isshown in FIG. 13( a)

Low-stress nitride (LSN) is used for isolation purpose; on top of it Ptis patterned by lift off; a thin layer of AlN is sputter deposited;afterwards the second Pt electrode is patterned by lift off and athicker AlN layer is sputtered deposit to form the second structurallayer. In FIG. 13( b) The Al top electrode is sputter-deposited andpatterned by dry etch in a Cl₂-based RIE system. In FIG. 13( c) contactsto the bottom and intermediate electrode are opened by wet etch of AlNin hot H₃PO₄. In FIG. 14( d) AlN is masked by low-temperature oxide(LTO) and dry etched in a Cl₂-based RIE system. LTO and LSN are thenetched and removed by dry etch. The final structure is dry released inXeF₂. Alternatively, the structures can be released by a wet etchingprocess. For the intermediate Pt electrode a lift off process wasperformed with the attention to use an ion-free developer that would notattack the AlN films. All the steps involved in the fabrication processoccur at low temperature (T_(max)<400° C.), therefore making these microdevices potentially post-CMOS compatible. Also, this two-layer structurerequires two subsequent depositions of AlN films. Highly polycrystallineAlN piezoelectric films were sputtered-deposited using an AMS singlemodule tool and showed a good rocking curve values of ˜1.3°.

Another aspect of the embodiments of the present invention is directedtowards the patterning of the electrodes and the effects of thosepatterned electrodes on the contour mode vibrations of the resonators.For example, in the resonators the portions that are used for applyingan alternating electric field include electrical interconnects between apatterned region or regions of the top electrode and a patterned regionor regions of the bottom electrode. In accordance with this aspect, anovel design for piezoelectric contour mode MEMS plate resonators isprovided that effectively uncouples the resonant frequency of thedevices from their overall dimensions by selectively patterning thetransduction electrodes and routing the excitation waveform as seen inFIG. 14. FIG. 14 shows an optical micrograph of micromechanicalresonator with selectively patterned half-wavelength period electrodes.The polarity of the electric excitation signal alternates betweenadjacent top and bottom electrode pairs. The effective characteristiclengths for determining frequency and motional resistance are 6 and 900μm, respectively. The ability to scale the lateral dimensions of thestructural material enables more mechanically robust devices that arecapable of attaining higher frequencies with reduced motionalresistances (e.g., 24Ω) and relaxed fabrication tolerances (structuralplate measures 51×100 μm). The electrode configuration can also suppressthe appearance of any lower or sub-harmonic modes in the frequencyresponse of the device.

In the patterned electrode embodiment, the frequency determiningdimension is effectively uncoupled from the overall dimensions of theAlN plate by patterning the transduction electrodes such that thepolarity of the excitation electric field alternates at half-wavelengthintervals in the direction of wave propagation as seen in FIG. 15. FIG.15 shows a schematic of resonator cross-section showing selectiveelectrode patterning and signal routing. The polarity of each top andbottom electrode pair alternates at half-wavelength intervals. Theperiod of the electrode pairs determines the operating frequency of theresonator while the number of periods affects its motional resistance.The polarity of the electrodes uniquely matches that of the strain fieldfor a specific bulk mode of vibration of the plate. Moreover, the numberof half-wavelength electrodes can be scaled to engineer a proportionaldecrease in motional resistance (the motional resistance also depends onthe length of the plate).

A practical implication of this aspect is that half-wavelength featuresneed only be defined in the thin metal electrodes, and the feasibilityof fabricating such features in a production environment has essentiallybeen proven by SAW device manufacturers (in fact SAW devices have moredemanding fabrication requirements due to their lower acousticwavespeed). As with SAW devices, the nominal frequency of the resonatordepends not only on the absolute dimensions but also the periodicity ofthe electrodes.

All publications, and descriptions mentioned above are hereinincorporated by reference in their entirety for all purposes. None isadmitted to be prior art.

As will be understood by those skilled in the art, the present inventionmay be embodied in other specific forms without departing from theessential characteristics thereof. For example, the geometry ormaterials for the top and the bottom electrodes may be the same ordifferent, and either one may be produced from aluminum, platinum,tungsten, molybdenum, ruthenium, chrome, gold, titanium dopedpolycrystalline silicon, and combinations thereof. These otherembodiments are intended to be included within the scope of the presentinvention, which is set forth in the following claims.

What is claimed is:
 1. A micromechanical piezoelectric resonator,comprising: a first electrode; a second electrode; a piezoelectric layerdisposed between the first electrode and the second electrode, whereinthe piezoelectric resonator comprises a planar surface and wherein thepiezoelectric resonator is configured to undergo in-plane lateraldisplacement in response to application of an electric field to thepiezoelectric layer by the first and second electrodes; and first andsecond effective quarter-wavelength anchors coupling the piezoelectricresonator to a substrate at first and second sides of the piezoelectricresonator, respectively, and wherein the first and second sides aresubstantially opposite each other.
 2. The resonator of claim 1 whereinat least one of the first electrode or the second electrode islithographically patterned to excite a specific region or regions of thepiezoelectric resonator.
 3. The resonator of claim 2 wherein the firstelectrode and the second electrode are lithographically patterned toexcite a specific region or regions of the piezoelectric resonator. 4.The resonator of claim 1 wherein said piezoelectric layer is a layerselected from the group consisting of aluminum nitride, gallium nitride,aluminum gallium arsenide, zinc oxide, quartz, zinc-sulfide,cadmium-sulfide, lithium tantalate, lithium niobate, and combinationsthereof.
 5. The resonator of claim 1 wherein at least one of the firstelectrode or second electrode is made from a material selected from thegroup consisting of aluminum, platinum, tungsten, molybdenum, ruthenium,chrome, gold, titanium, doped polycrystalline silicon, niobium, copper,silver, tantalum, cobalt, nickel, palladium, silicon germanium, dopedconductive zinc oxide, and combinations thereof.
 6. A device comprisinga plurality of resonators electrically cascaded to form a network, theplurality of resonators including the resonator of claim
 1. 7. Thedevice of claim 6 wherein said network is a Pi network, T network, Lnetwork or a lattice network.
 8. The device of claim 7, wherein theresonator forms at least part of a band pass filter circuit formed froma series of networks comprising the network.
 9. The device of claim 7,wherein at least two resonators of the plurality of resonators areelectrically connected.
 10. The device of claim 7 comprising electricalconnectors used to route a signal from a first resonator of theplurality of resonators to a second resonator of the plurality ofresonators.
 11. The resonator of claim 1, wherein the resonator is oneof a plurality of resonators arranged in combination as a band-passfilter circuit.
 12. The resonator of claim 1 wherein the resonator is apart of a frequency reference in an oscillator circuit.
 13. Theresonator of claim 1, further comprising electrical interconnectsbetween a patterned region or regions of the first electrode and apatterned region or regions of the second electrode.
 14. The resonatorof claim 1, wherein the piezoelectric layer is configured to undergoin-plane lateral displacement in response to application of the electricfield to the piezoelectric layer by the first and second electrodes.